Hard X-rays from Emission Line Galaxies and the X-ray Background: A Test for Advection Dominated Accretion with Radio Sources
Yi, Insu · Boughn, Stephen P.
Original · EN
Recent studies of the cosmic X-ray background (XRB) have suggested the possible existence of a population of relatively faint sources with hard X-ray spectra; however, the emission mechanism remains unclear. If the hard X-ray emission is from the radiatively inefficient, advection dominated accretion flows (ADAFs) around massive black holes in galactic nuclei, X-ray luminosity and radio luminosity satisfy the approximate relation Lᵣ 7× 10³⁵(ν/15GHz)⁷/⁵(M/10⁷M) (Lₓ/10⁴⁰ erg s⁻¹)¹/¹⁰ erg s⁻¹ where Lᵣ=νLν is the radio luminosity at frequency ν, M is the mass of the accreting black hole, and 10⁴⁰ Lₓ 10⁴² erg s⁻¹ is the 2-10 keV X-ray luminosity. These sources are characterized by inverted radio spectra Iν∝ ν²/⁵. For example, an ADAF X-ray source with luminosity Lₓ 10⁴¹ erg s⁻¹ has a nuclear radio luminosity of 4× 10³⁶(M/3× 10⁷M) erg s⁻¹ at 20 GHz and if at a distance of 10 (M/3× 10⁷M)¹/² Mpc would be detected as a 1mJy point radio source. High frequency (20 GHz), high angular resolution radio observations provide an important test of the ADAF emission mechanism. Since Lᵣ depends strongly on black hole mass and only weakly on X-ray luminosity, the successful measurement of nuclear radio emission could provide an estimate of black hole mass. Because the X-ray spectra produced by ADAFs are relatively hard, sources of this emission are natural candidates for contributing to the hard, >2 keV, background.
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